This invention relates generally to a remote smoke alarm station and more particularly it relates to such a station for use in a fire alarm system having a central control station and a plurality the remote smoke alarm stations each connected to the control station by a pair of direct current supply wires for feeding an ionization sensor of a smoke detecting circuit. Each detecting circuit includes a measuring ionization chamber connected in series via a common intermediate electrode with a reference ionization chamber; the intermediate electrode is connected to a gage electrode of a field effect transistor the output of which is connected to an electronic switching circuit having two stable states, namely a high ohmic state corresponding to a ready condition and a low ohmic state corresponding to an alarm condition which is signaled via the supply wires into the central station.
A fire alarm system of this kind is known from the German publication DT-OS No. 2,261,179. In order to avoid false alarms resulting from the activation of the alarm station caused by the response of the alarm station to interferences in the supply wires, each alarm station includes a control pulse generator and a resetting pulse generator for feeding an electronic switching circuit made for example in the form of a feedback switching amplifier having two stable states. This known system, however, does not make possible to test the operative condition of the respective alarm stations from the central station in spite of relatively complex circuitry in each alarm station. Since the alarm systems of this type have frequently an extremely large number of remote alarm stations, the cost of this system is relatively high.
From the German Patent DT-PS No. 1,566,687, a fire alarm system is also known which has parallel connected ionization alarm stations fed from corresponding direct current supply lines and having an electronic switching circuit which stores the alarm indication, that means, it remains activated even when the alarm releasing conditions have ceased to exist. In this system it is possible to extinguish from the central control station the stored alarm conditions in respective alarm stations by simply interrupting the supply voltage from the signaling and supply lines. However, when the operative condition of the alarm stations is to be tested from the central station, there is necessary to provide a third testing conduit in addition to the two supply conduits and moreover an indication circuit in the central station is necessary.
Finally, from the German publication DT-AS No. 1,766,440, a fire alarm system is known which permits the testing of working condition of respective alarm stations from the central station without the use of the third testing conductor. In this system, each signaling and supply line is not fed by a direct current voltage but instead, is fed by an alternating current voltage and each alarm station is designed such as to have a high impedance for one polarity of the supplied voltage and a low impedance for the other polarity of the supply voltage whereby in the case of a fire alarm the two impedances are reversed. The central station is provided with means for separate reading of the impedance between the conductors of a line during the change of the two polarity signs of the supplied voltage and thus distinguish between the activation caused by the alarm or by the interference. This arrangement, however, necessitates again a substantially increased cost of the circuitry involved; in addition such a system employing an alternating current for feeding the remote alarm stations is more sensitive to interference or false signals induced into the supply line than the system using alarm stations fed with a direct current voltage.